Image sensors with an increasing number of image sensing photo-detector elements are being used in digital cameras designed to be sold in large quantities to consumers. Although such cameras most commonly utilize sensors having from 2 to 4 megapixels at the present time, it is not uncommon to find digital cameras available that employ image sensors as large as large as 5 to 6 megapixels. And this number will likely increase in the future. Normally, not all the light sensing elements in these sensors work correctly; some are defective. Defective light sensing elements do not react to light properly, and thus distort a resulting image. In some cases, one or more light sensing elements may always produce an output, even though light is not present. In other cases, a few may produce no light output even when very bright light impinges on their surface. The result can be a visible bright or dark spot in the captured image, respectively. In a third case, a light output is produced which is responsive to the light impinging on a sensor's surface, but this output is not correctly related to the light falling on the sensor. These pinpoint imperfections may be isolated or found in clusters. Since each pixel in the usual sensor captures information associated with different colors of the scene being photographed, these defective pixels are often visualized as pixels of the wrong color, not just pixels which are too light or too dark.
A great deal of time and cost is expended in sorting out image sensors with these imperfections. In order to be able to use as many of these partially defective sensors as possible, electronic defective pixel correction techniques are used. Such techniques employ a predefined list of locations which identify where the defective pixels are located in the sensor. The locations of these defective pixels are obtained by using a time consuming calibration procedure on each sensor. This procedure usually employs measuring the signal output of the sensor pixels with a black and then a white image incident on the sensor. For the black image, all light is blocked from the sensor and pixels with a higher output above a set threshold are identified as defective. Similarly, for the white image, a uniform illumination field is allowed to fall onto the sensor in order to identify any defective pixels that generate a lower output signal above a specified threshold. Any such pixels are considered “exceptional” and are included in a defective pixel list for each sensor that is tested. The list includes the locations in the array of the defective pixels.
When such a calibrated image sensor is installed in a digital camera or other host device, its defective pixel list is loaded into a memory of the digital camera in the form of a look-up table. During image processing, the digital camera's processor searches this lookup table to determine if a particular pixel is defective and therefore needs to be corrected. This correction usually takes the form of replacing the output from a defective pixel with a simple average of the outputs of good pixels adjacent to the defective pixel. As the number of pixels in a sensor becomes larger, the size of the memory necessary to store the look-up table increases as well as the time required to do the look-up and then calculate the average of the adjacent pixels.